Support for reusable InstrumentValueArea control

Fix sizeToContents support

Custom combo box was not using our own custom font. As a result, it would respect neither the font family nor font size accordingly.

Reduce height of items in combo dropdown

Tweak combobox visuals

Update for Qt 5.11

New features:
* Less padding to the right of drop indicator
* Ability to set font
* alternateText - Set alternate text for button display
* sizeToContents - Width will adjust to fit all contents

Fix dropdown indicator color

Fix bug where QGCComboBox { model: ListModel would not work correctly

Update to the QGCComboBox to use a textRole in the case where it's availble.

Some small theme colour fixes.

Updating the items to be inline with the theme.

Fix height

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

On 5.7 and 5.8 somethings this would happen:
* The menu would be ordered incorrectly due to bug in Instantiator
* The menu would be positioned incorrectly

Also correctly set implicitHeight on QGCTextField